This invention relates to an improved process for producing 2-epi-fortimicin A directly from fortimicin A.
2-epi-fortimicin A is an aminoglycoside antibiotic disclosed in commonly assigned U.S. patent application Ser. No. 79,130, filed September 26, 1979, now U.S. Pat. No. 4,331,804. This compound can be represented by the following structural formula: ##STR2##
Prior to the present invention, 2-epi-fortimicin A was obtained by a long and tortuous process through the selective epimerization of fortimicin B at the 2-position and the subsequent conversion of 2-epi-fortimicin B to 2-epi-fortimicin A.
Generally, in order to selectively epimerize the 2-position of fortimicin B, it was necessary to first prepare 1,2',6'-tri-N-benzyloxycarbonylfortimicin B-4,5-carbamate which was then converted to the 2-O-methanesulfonate intermediate. Solvolysis of the 2-O-methanesulfonate intermediate in the presence of ammonium acetate resulted in an approximately equimolar mixture of the tri-N-benzyloxycarbonyl-2-epi-4,5-carbamate an the 2',6'-di-N-benzyloxycarbonyl-2-epi biscarbamate. Alternatively, solvolysis of the 2-O-methanesulfonate intermediate in a mixture of tetrahydrofuran and sodium bicarbonate resulted in the 2-epi-oxazoline intermediate. Heating of the latter intermediates under reflux in a solution prepared from ammonium acetate and an aqueous 1,2-dimethoxyethane resulted in an approximately equimolar mixture of the 2-epi-4,5-carbamate intermediate and the 2-epi-biscarbamate intermediate.
The mixture of the 2-epi-mono and biscarbamates were then separated into pure components by chromatography or heated under reflux with a mixture of sodium bicarbonate and methanol to convert the monocarbamate to the biscarbamate which is then isolated by chromatography.
Hydrogenolysis of the 1,2'-di-N-benzyloxycarbonyl-2-epi-biscarbamate with 5% palladium on carbon in the presence of methanolic hydrochloric acid resulted in 2-epi-fortimicin B dihydrochloride. Incomplete hydrolysis results in a mixture of 2-epi-1,4-urea and the desired 2-epi-fortimicin B and care had to be taken to employ a sufficiently long hydrolysis time.
2-epi-Fortimicin B was then converted to 1,2',6'-tri-N-fortimicin B with N-benzyloxycarbonylsuccinimide. 4-N-acylation of the N-protected intermediate with the N-hydroxysuccinimide ester of N-benzyloxycarbonylglycine resulted in tetra-N-benzyloxycarbonyl-2-O-[N-benzyloxycarbonylglycyl]-2-epi-fortimicin A as the major product and the desired tetra-N-benzyloxycarbonyl-2-epi-fortimicin A is the minor product. Catalytic hydrogenations of the latter compounds with 5% palladium on carbon in 0.2 N-methanolic hydrochloric acid resulted in 2-O-glycyl-2-epi-fortimicin A and 2-epi-fortimicin A, respectively.
Since 2-epi-fortimicin A was obtained as the minor product in the above prior art synthesis, an alternate method of preparing the compound was sought. In that method, fortimicin B was converted to tetra-N-acetylfortimicin B and selectively hydrolyzed to provide 1,2',6'-tri-N-acetylfortimicin B. The latter was converted to the 4-N-ethoxycarbonyl derivative which was then cyclized to the 4,5-carbamate in a refluxing suspension of sodium bicarbonate in aqueous methanol. Treatment of the latter with methanesulfonic acid in pyridine resulted in the 2-epi-1,2-oxazolidine intermediate. Hydrolysis with aqueous hydrochloric acid in tetrahydrofuran resulted in 1,2',6'-tri-N-acetyl-2-epi-fortimicin B-4,5-carbamate. The latter was then converted to the corresponding 2-O-benzyl ether with benzylbromide in N,N-dimethylformamide in the presence of barium oxide and barium hydroxide. Hydrolysis with aqueous sodium hydroxide resulted in 2-O-benzyl-2-epi-fortimicin B. The latter was then treated with N-benzyloxycarbonylsuccinimide to provide 1,2',6'-tri-N-benzyloxycarbonyl-2-epi-fortimicin B. Treatment of the latter with the N-hydroxysuccinimide ester of N-benzyloxycarbonylglycine resulted in the tetra-N-benzyloxycarbonyl-2-O-benzyl-2-epi-fortimicin A intermediate. Catalytic hydrogenolysis of the latter in the presence of 5% palladium on carbon finally resulted in the desired product.
In contrast to the foregoing relatively long and ardous methods of obtaining 2-epi-fortimicin A through 2-epi-fortimicin B, the present invention provides a much simplified process of obtaining 2-epi-fortimicin A directly from fortimicin A by protecting the amine groups of fortimicin A, converting the N-protected fortimicin A to an intermediate of the formula: ##STR3## wherein L is a leaving group and Z is an amine-protecting group, reacting the intermediate (I) with a loweralkyl metal halide in N,N-dimethylformamide to form N-protected-2-epi-fortimicin A, and then removing the N-protecting groups to obtain 2-epi-fortimicin A.